#removedupes-button {
  list-style-image: url("chrome://removedupes/content/removedupes-button-24x24.png");
  -moz-image-region: rect( 0px 24px 24px 0px);
}

[iconsize="small"] #removedupes-button {
  list-style-image: url("chrome://removedupes/content/removedupes-button-16x16.png");
  -moz-image-region: rect( 0px 16px 16px 0px);
}

/* no special image for

#removedupes-button:hover

and
[iconsize="small"] #removedupes-button:hover

for now
*/